1. Field of the Invention
This invention relates generally to integrated circuits and in particular to floating gate transistors.
2. Description of the Related Art
Programmable memories and logic circuits are integral parts of a digital system, such as a computer, and can have similar physical structures. One type of programmable memories is an electrically erasable and programmable read only memory (EEPROM), which is a reprogrammable nonvolatile memory that is widely used in the computer system for storing data both when power is supplied or removed. The typical data storage element of an EEPROM is a floating gate transistor, which is a field-effect transistor (FET) having an electrically isolated (i.e., floating) gate that controls electrical conduction between source and drain regions. Data is represented by charges stored on the floating gate and the resulting conductivity obtained between the source region and the drain region.
Recently, logic circuits based on programmable memories have been introduced. The logic circuits are implemented by field programmable logic arrays (FPLAs) which provide a flexible architecture via user-programmed on-chip fuses (i.e., switches) to perform specific functions for a given application. The FPLAs are fabricated with floating gate transistors similar to the programmable memories. However, the floating gate transistors in the FPLAs act as switches rather than as storage elements. A common problem among the programmable memories and the related FPLAs is a large cell size, which limits circuit density.